The present invention relates to a motor controller and an electric power steering apparatus having the motor controller.
In many cases, a motor controller provided in an electric power steering apparatus (EPS) includes anomaly detecting means that detects an anomaly such as a failed electric current flow caused in any one of phases of a motor, that is, U, V, or W-phase, due to a break of a power supply cable or a failed contact of a driver circuit. Normally, if an anomaly is detected by the anomaly detecting means, the motor is quickly stopped to perform fail-safe.
This greatly changes steering characteristics of the EPS, requiring the driver to increase the force applied to the steering wheel. In this regard, for example, Japanese Laid-Open Patent Publication No. 2003-26020 discloses a motor controller that continuously operates a motor using two phases other than a phase with a failed electric current flow even after such failure has been detected in the phase. In this manner, assist force is continuously applied to a steering system so as to assist in manipulation of a steering wheel by the driver. This suppresses increase of load on the driver.
However, if a sinusoidal wave electric current is supplied to the respective U, V, and W-phases of the motor and the motor is continuously operated using the two phases other than the phase with the failed electric current flow, as has been described, after such failure has occurred in any one of the phases (which is, for example, the U-phase, as illustrated in FIG. 10), a torque ripple is caused and steering comfort is decreased due to the torque ripple. Specifically, in this case, the motor is continuously operated without obtaining a motor electric current matching a required torque. In other words, with reference to FIG. 11 in which change of the motor electric current in a two phase drive mode is represented in a d/q coordinate system, the actual q-axis electric current value changes in a sinusoidal manner despite a constant q-axis electric current command value, which is a target control value of the motor torque.